The present invention relates to electrolytic aluminum production cells, and more particularly relates to systems for improving alumina distribution in such cells by controlling the flow patterns of oxygen bubbles generated during the aluminum production process.
Electrolytic cells of the Hall-Heroult type are used to smelt alumina ore into aluminum metal. These cells use consumable carbon anodes which have several disadvantages. For instance, the CO2 released by the reaction of the carbon with oxygen from the alumina ore may cause environmental problems. Also, because the anodes are consumed they must be replaced every 3-4 weeks. Replacing an anode disrupts cell operation due to the associated cooling effects, electrical imbalance and release of fluoride emissions. Environmental emissions are associated not only with the use of carbon anodes, but with the production of carbon anodes as well.
Carbon anodes can be replaced by inert anodes that release O2 instead of CO2, do not require changing and can be manufactured in an environmentally-friendly way. A promising inert anode material is a cermet consisting of an oxide matrix based on nickel-iron-ferrite or nickel ferrite, and other additives surrounding a highly conductive, metal phase containing Cu, Ag and other additives. Although the oxides in the cermet may be soluble to some extent in the cryolite-based electrolyte (bath) and may be reduced by aluminum metal, it is possible to reduce anode corrosion to a very low level so that the anodes are essentially inert. One approach for reducing corrosion rate is to maintain the concentration of alumina in the bath at or near saturation. It is especially important to avoid alumina depletion near the active surfaces of the anodes.
Achieving a high and uniform alumina concentration adjacent to the anodes requires optimization of two processes, alumina dissolution and alumina distribution. In the dissolution process cold alumina is fed to a cell and molten bath freezes on the alumina grains. Heat must be supplied to raise the temperature of the alumina to the cell temperature and melt the frozen bath. Then additional heat of solution must be supplied. This process is limited by heat transfer because, in order to increase current efficiency, cells are run with a small superheat. The dissolution process also involves mass transfer between the bath near the alumina grains and bulk bath. These have compositions of Csaturation and Cbulk. The rate depends on (Csaturation-Cbulk). This difference approaches zero in the inert anode cell run near saturation.
If alumina does not completely dissolve, the excess tends to form muck or sludge, a mixture of bath and undissolved alumina that collects on the bottom of the cell under the metal layer or xe2x80x9cpadxe2x80x9d. This muck causes maldistribution of current, leading to a xe2x80x9cnoisyxe2x80x9d cell that tends to run at higher voltage and lower current efficiency than a cell without muck. The muck often forms hard deposits on the cell bottom which are difficult to remove and perpetuate the noisy condition. Modern Hall-Heroult cells are normally run xe2x80x9cleanxe2x80x9d, i.e., the alumina concentration is relatively low and far from saturation. A lean operation reduces the amount of undissolved alumina in the cell because the driving force for dissolution is increased. Alumina concentration is typically more uniform in a xe2x80x9cleanxe2x80x9d cell than in a xe2x80x9crichxe2x80x9d cell.
Once the alumina dissolves, the enriched bath must be distributed throughout the cell to feed the anode reaction. In modem pre-bake carbon cells, alumina is usually added to a few locations called point feeders. In conventional aluminum smelting cells with carbon anodes, alumina distribution is relatively slow. Based on water model tracer tests, it has been observed that, after feeding a shot of alumina through a point feeder, it may take 0.5-1 hour for the bath concentration to become uniform. Bubble-driven flow creates circulation locally around each anode but does not drive large-scale circulation in the cell. Overall cell circulation is driven mainly by electromagnetic forces and turbulent diffusivity. Cell designers have traditionally attempted to minimize these forces because they adversely affect power efficiency.
Since an inert anode cell is preferably run xe2x80x9crichxe2x80x9d in alumina to avoid corrosion of the anodes, suitable means must be used to enhance dissolution and distribution in order to avoid muck and to insure a uniformly high concentration of alumina in the bath adjacent to the anodes. Examples of such means are the use of high-surface-area, gamma alumina that dissolves readily, preheating the alumina to reduce thermal requirements, and feeding continuously at optimized feeder locations. In spite of the application of these and other methods, muck is a problem in inert anode cells.